The herpesviruses have co-evolved with their hosts for hundreds of millions of years. As our adaptive immune systems evolved, so did the mechanisms of immune evasion encoded by herpesviruses. Because herpesviruses remain latent or persistent within their host throughout life, the herpesviruses have evolved a unique set of strategies to help them contend with the lifelong host immune response. This application's central premise is that the herpesviruses, in their evolution, have performed a 100 million year- long genetic screen for mechanisms of immune evasion. We seek to take advantage of their evolution to discover new concepts in cell biology, immunology and virology. The long term goal of this project is to understand the mechanisms by which human herpesviruses evade detection by the immune system. The focus of this proposal is to develop a humanized mouse model system for the study of human herpesviruses-6 and -7 (HHV-6/7). We have found that one HHV-6/7-encoded orf, U21, associates with and downregulates class I MHC molecules from the cell surface, presumably as a means of escaping cytotoxic T cell detection. In addition, U21 also downregulates the NK activating ligands MICA and MICB from the cell surface, and expression of U21 reduces NK cytotoxicity toward U21-expressing cells. HHV-6/7 are highly specific for their human hosts, so there is no animal model system available. Moreover, a genetically manipulable BAC system has been difficult to create. Thus, while we have demonstrated these functions of U21 in vitro, when U21 is expressed in tissue culture cells, we do not yet understand the relative contributions of these immunoevasin molecules in the context of virus infection. Recently, a BAC system for the study of HHV-6 was successfully developed. Using a ?U21 HHV6 virus, we will be able to examine the influence of U21 upon class I MHC molecules and NK activating ligands in the context of virus infection. Then, to determine the impact of U21 on the pathogenesis of HHV-6 infection, we will infect humanized scid-hu mice with recombinant HHV-6. The proposed studies will also result in better understanding of acute and long-term HHV-6 infection. A better understanding of these understudied viruses is essential: not only will greater knowledge of these viral mechanisms lead to a more thorough comprehension of how these viruses have such great success in evading the host immune response, but a greater understanding of these viruses may have future clinical application in the treatment of epilepsy, transplant recipients, and autoimmune disorders. RELEVANCE OF THIS RESEARCH TO PUBLIC HEALTH The high fevers resulting from HHV-6 infection are perhaps the most common cause of febrile seizures in young children, with a peak incidence in the second year of life. Childhood seizures have long been known to be associated with a substantially increased risk of temporal epilepsy (3). A recent study has shown that HHV- 6, and HHV-7 infection accounts for ~40% of all cases of severe seizures in young children (4). HHV6 DNA was found in 70% of brain tissue resections from mesial temporal lobe epilepsy (5). Together, these studies suggest a link between childhood infection with HHV-6 and -7, and the development of epilepsy. HHV-6 also reactivates in hematopoietic cell transplant recipients, resulting in poor neurocognitive outcome and can result in acute limbic encephalitis (3,4).